Paypal add registration form not redirecting to paypal.

This is my first time using Gravity Form,
so i'm sorry before if the problem is from my settings.
But several others also have the save problem.

http://www.gravityhelp.com/forums/topic/latest-version-of-paypal-add-on-can-it-cause-problems#post-204761

I use gravity form to create subscription + member registeration.
The addon i use: GF Paypal Add-on (1.7) and User Registration Add-on (1.5).

the problem is when user submit form it didn't redirect to paypal.

Right now in the site registration/order page i switch back using eShop plugin.
but the page to GF Form is still accessible:
http://autohosted.com/buy/

thanks